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Tribology Transactions
ISSN: 1040-2004 (Print) 1547-397X (Online) Journal homepage: http://www.tandfonline.com/loi/utrb20
A Model for Shear Degradation of Lithium Soap
Grease at Ambient Temperature
Yuxin Zhou, Rob Bosman & Piet M. Lugt
To cite this article: Yuxin Zhou, Rob Bosman & Piet M. Lugt (2018) A Model for Shear
Degradation of Lithium Soap Grease at Ambient Temperature, Tribology Transactions, 61:1, 61-70, DOI: 10.1080/10402004.2016.1272730
To link to this article: https://doi.org/10.1080/10402004.2016.1272730
© 2018 Society of Tribologists and Lubrication Engineers© Society of Tribologists and Lubrication Engineers Accepted author version posted online: 21 Dec 2016.
Published online: 13 Feb 2017. Submit your article to this journal
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A Model for Shear Degradation of Lithium Soap Grease at Ambient Temperature
Yuxin Zhoua, Rob Bosmana, and Piet M. Lugta,ba
University of Twente, Enschede, The Netherlands;bSKF Engineering & Research Centre, Nieuwegein, The Netherlands
ARTICLE HISTORY
Received 21 June 2016 Accepted 12 December 2016
ABSTRACT
In this article, the shear degradation of lithium 12-hydroxy stearate grease will be measured using an in-house-developed Couette aging machine. In this device the shear rate is well defined. The aging is related to the generated entropy density as described in Rezasoltani and Khonsari’s work (Tribology Letters, Vol. 56, No. 2, pp. 197–204, 2014). The rheological properties of the aged samples were evaluated using a parallel-plate rheometer. The results showed that there are two aging phases with different degradation rates: a progressive degradation phase at the early stage, followed by a rather slow deterioration afterwards. Based on this observation, an aging equation was formulated to describe the aging behavior of lithium-thickened grease. Atomic force microscopy results of the fresh and aged greases showed that the variation in thickener microstructure provides a good explanation for the lithium grease degradation mechanism: under shear, the original fibrous network is progressively destroyed and becomes fragmented, leading to the loss of consistency and a change in the rheological properties.
KEYWORDS
Grease degradation; rheology; microstructure
Introduction
Grease is a widely applied lubricant mostly used in rolling bear-ings. It is a multiphase system consisting of three parts: thick-ener (3–30%), base oil (70–90%), and additives (Lugt(1)). As a semisolid material, grease has a high consistency, which pre-vents leakage and creates a reservoir of lubricant inside the bearing. However, when subjected to the severe conditions within a rolling bearing, grease will undergo high shear, possi-bly causing deterioration. The degradation of this grease is usu-ally reflected by the loss of its original consistency (softening), possibly yielding leakage from the bearing and, hence, starva-tion. It may also lead to continuous churning and high temper-ature. Both cases result in a reduced life of the bearing (Lugt
(1)). It is therefore valuable to investigate the mechanism of grease degradation and to develop predictive models for this.
Generally, the degradation of grease is classified as chemical or mechanical aging. This article focuses on the mechanical aging of lithium 12-hydroxystearate–thickened grease. This type of grease takes the major share of the worldwide industrial grease market and is widely applied in rolling bearings due to its wide temperature applicability, relatively good mechanical stability, water-resistant properties, and low cost (Lugt(1)).
The most straightforward way to study grease aging is to obtain data fromfield tests directly, where the grease is worked in a real bearing (Salomonsson, et al.(2)). Suchfield practice requires a long timescale study. For example, in the work of Lundberg and H€oglund(3), aged samples were collected from the wheel bearings of railway wagons after years of service. Another drawback of this method is the fact that it is practically impossible to estimate the
exact aging conditions—that is, the shear stress/rate, temperature, and time—to which the sample was subjected during operation.
Currently, there are two standards for measuring grease degra-dation. One is the mechanical stability test where normal load and shear are applied; this is considered particularly meaningful in the situation where the bearing is subjected to vibrations and where the grease is continuously thrown back into the tracks. A typical mechanical stability test is the roll stability test (ASTM D 1831), where grease is sheared between a heavy roller (with a lead core) and a hollow rotating cylinder at an elevated temperature (gener-ally 80C). It was found that this test can be used to simulate the practical working conditions in automobile wheel bearings (Bondi, et al.(4); Moore and Cravath(5)), and rolling bearings in railway wagons (Lundberg(6); Lundberg and Berg(7)). The other test is the shear stability test, where only shear is applied. This is considered important for bearings running under relatively stable working conditions. When subjected to continuous shear, it is observed that shear degradation of the grease results in the release of oil and thus provides lubricant replenishment (Merieux, et al.
(8); Cann, et al.(9)). In the“grease worker” (ASTM D 217), which consists of a closed cylinder and a piston plate with a number of holes, the grease is sheared through the holes during a well-defined number of strokes (usually 10,000 or 100,000 strokes).
The drawback of the two ASTM aging methods mentioned above is that the applied shear condition is not well defined, which makes it difficult to use these methods for the develop-ment of predictive aging models. To measure aging as a function of shear and time, a modified Couette rheometer was used in the shear measurements of grease by Paszkowski (10). Rezasoltani
CONTACT Yuxin Zhou y.zhou@utwente.nl
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TRIBOLOGY TRANSACTIONS 2018, VOL. 61, NO. 1, 61–70
and Khonsari(11)made use of a parallel-plate rheometer for the long-term shear tests of lithium-thickened grease. Aging in a rhe-ometer provides a controlled aging process, where the rheology can directly be measured as a function of time. However, the dis-advantage of a parallel-plate configuration is that the shear field within the gap is not uniform, resulting in an inhomogeneous aging condition. In addition, in the current rheological study, leakage was observed in the parallel-plate geometry due to the centrifugal forces and in the open Couette configuration due to the Weissenberg effect (Schramm (12)). Hence, at some point during aging, the measurement will become inaccurate. There-fore, more robust test rigs are required.
Rezasoltani and Khonsari(11)discovered a linear relation-ship between the energy input and the grease properties during grease aging tests using a parallel-plate rheometer. They men-tioned that this“linear correlation remains valid regardless of the applied shear rate or the grease temperature” (p. 200). This was verified using a journal bearing mounted between two roll-ing bearroll-ings to provide a uniformfilm along the circumference of the journal bearing (hence the journal bearing was not loaded) and a modified grease worker. The experimental data of the journal bearing test rig showed a slight deviation from the linear relation obtained from the rheological measurements at the end of their experiments, which was ascribed to grease separation from the journal and a slippage effect during the tests.
The aim of the current study is twofold: thefirst objective is to follow up on the work of Rezasoltani and Khonsari(11). The influence of grease mechanical degradation on its shear stability will be evaluated under similar aging conditions using lithium-thickened grease samples but with an increased aging period,
hence increasing the total amount of entropy generated. The second goal is to study the underlying mechanism responsible for the aging of lithium grease.
To achieve thefirst task, fresh greases will be sheared in an in-house-made Couette aging machine at specific shear rates for a set period of time. Then the grease is sampled and its rhe-ological properties are measured. Though lubricating grease can be considered chemically stable at low temperatures (Ito, et al. (13), Fourier transform infrared spectroscopy (FTIR) measurements will be performed for the fresh and aged samples to confirm this. The physical processes occurring during mechanical degradation will be studied by measuring the change in the microstructure of the aged samples using atomic force microscopy (AFM).
Materials and method
Two commercial lithium-thickened greases, Li/M and Li/SS, were used. Information on their composition is presented in
Table 1(Cyriac, et al.(14)). Aging tests
Test rig
In the current study, the grease has been aged by means of applying specific shear rates for a set period, similar to what is done in a grease worker. However, the shear rate is now well defined. Normal load (or hydrostatic pressure) will not be applied atfirst (so only the shear stability of the grease will be studied). The grease should be aged in a closed system, where leakage is avoided. Another requirement for the new test rig is Nomenclature
F Average load per stroke inside the grease worker (N) f Frequency for the oscillatory test (Hz)
G0 Storage modulus (Pa) G00 Loss modulus (Pa)
h Gap height of the aging machine (m) K Coefficient of degradation
L Average thickenerfiber length (mm)
Lpiston Piston displacement for one full stroke of the grease
worker (m)
m Degradation exponent R2 Goodness offit
Ra Surface roughness of the measuring plates (centre
line average, mm)
Ri Radius of the rotating bob (m)
Ro Radius of the stable housing case (m)
Sg Generated entropy per unit volume during aging
(J=mm3K)
_Sg Entropy generation rate per unit volume during
aging (J=mm3Ks )
Sgw Generated entropy per unit volume inside the grease
worker (J=mm3K)
Sps Generated entropy per unit volume during preshear
(J=mm3K)
Tgw Ambient temperature of the grease worker (K)
Tps Temperature during preshear (K)
Va Grease volume inside the Couette aging rig (mm3)
Vgw Grease volume inside the grease worker (mm3)
Wgw Work applied inside the grease worker (J)
Y1 Second stage rheological value after infinitely long aging Yi Initial rheological value for fresh grease
_ga Aging shear rate (s¡ 1)
_gps Shear rate for preshear (s¡ 1)
D Cone penetration depth (0:1 mm) h0 Zero shear rate viscosity (Pa¢ s)
h1 Grease viscosity after infinitely long aging (Pa ¢ s) hb Base oil viscosity (Pa¢ s/
hi Initial zero shear viscosity for fresh grease (Pa¢ s/ h j_ga Grease viscosity at the aging shear rate _ga(Pa¢ s)
t Shear stress within the aging gap (Pa) tc Crossover stress (Pa)
tps Shear stress during preshear (Pa)
ty¡ HB Yield stress obtained from the Herschel-Bulkley
model (Pa)
ty¡ OSC Yield stress obtained from the oscillatory strain sweep test (Pa)
’ Thickener volume fraction
that a sufficient amount of aged grease can be collected for sub-sequent rheological tests.
The new test rig (called a Couette aging test rig) is shown in
Figure 1. The basic concept is analogous to a cylindrical viscome-ter: the grease is to be sheared between a stationary housing case and a rotating bob, which is driven by a motor and a belt trans-mission. The shear rate exerted over the grease can thus be cal-culated based on the input rotational speed and the geometry of the aging head. The temperature during the aging procedure is captured by the thermocouple at the end of the aging head. To prevent grease leakage due to the Weissenberg effect, a lip seal is mounted on top of the aging gap. The rig was designed for a grease sample volume of VaD 5:1 £ 104 mm3for each aging test
(where RiD 40 mm; RoD 42 mm; h D 100 mm, leaving a
2-mm gap between the rotating bob and the housing case). Aging condition
The aging rotational speeds are selected such that the imposed shear rates are similar to those applied on the aging tests per-formed by Rezasoltani and Khonsari(11). The input rotational speeds, corresponding shear rates, and aging periods are listed inTable 2.
Sampling
The lip seal (seeFigure 1) generates a moderate amount of fric-tional heat, leading to a temperature gradient in the vertical
direction. Together with possible thickener–oil separation due to shear, the thickener of the aged grease might no longer be evenly distributed inside the aging gap. The aged samples were therefore mixed for 500 strokes in an in-house-made grease worker before the rheological measurements were performed.
Rheological measurements
Rheological measurements were performed for the fresh and aged grease samples using an MCR 501 Anton-Paar rheometer with parallel plate configuration. The viscosity was measured by steady-state flow curve measurements. In addition, oscil-latory strain sweep measurements were performed to measure the grease’s viscoelastic properties.
Preparation
There are three major concerns during rheological measure-ments: wall slip, loading history, and edge effects. To reduce the influence of wall slip, measuring plates with rough sur-faces are recommended (Czarny (15); Paskowski (16)). Therefore, the plates were roughened by sand-blasting (top plate: RaD 1:5 mm; bottom plate: RaD 2:3 mm). To minimize
the initial deviation induced by the placing and loading proce-dure, the grease samples were first deposited on the bottom plate and the top plate descended at a controlled speed until the measure position was reached, leaving a 1-mm measuring gap. Thereafter, preshear following a DIN standard (Deutsches Institut f€ur Normung(17)) was applied (preshear at _gpsD 100 s¡ 1for 60 s at 25℃). Subsequently, the
accumu-lated grease at the plate periphery was carefully removed with a spatula. The loading and pre-shear procedure is illustrated inFigure 2.
As a thixotropic material, regeneration of the thickener microstructure occurs after shearing the grease (Paszkowski
(16); Paszkowski, et al.(18)). Therefore, before data collection, sheared grease is left to rest for a sufficient relaxation time. This duration will depend on the grease microstructure, thick-ener concentration, preshear condition, etc., and can be deter-mined by a time sweep measurement.
Here, the time dependency of the shear modulus was recorded while imposing an oscillatory shear well within the linear viscoelastic regime (the applied shear is suffi-ciently small to not disrupt the grease properties). The detailed procedure is as follows: after preshear, a 2-h Table 1.Composition and properties of the greases studied.
Grease NLGI Thickener
Volume fraction of the thickener ’
Shape and average
size of thickener Base oil
Base oil viscosity at 25℃ hb Li/M 3 Lithium 12-hydroxy stearate 14% TwistedfiberL 2 mm Mineral oil 0:23 Pa ¢ s Li/SS 2 Lithium 12-hydroxy stearate 16% TwistedfiberL 2 mm Semi-synthetic 0:07 Pa ¢ s
Figure 1.Couette aging test rig.
Table 2.Aging condition for Li/M and Li/SS. Rotational speed (rpm) Shear rate _ga(s¡ 1) Aging time (h) 83 174 5 25 50 100 200 125 261 5 25 50 100 200 166 348 5 25 50 100 200 TRIBOLOGY TRANSACTIONS 63
oscillatory test was applied at a constant shear stress of 10 Pa, oscillation frequency fD 1 Hz, and temperature 25§ 1℃. Both Li/M and Li/SS show a similar trend: the high-est recovery of G0takes place during thefirst hour of relaxa-tion and the value levels out afterwards, which is in agreement with the literature (Paszkowski, et al.(18)).
As a consequence, a relaxation time of 60 min was applied prior to the tests. The application of pre-shear and sufficient relaxation guarantees that the deviation in the following rheo-logical results can be controlled within a 10 % spread, which satisfies the requirements for grease rheological measure-ment specified in DIN 51810-2 (Deutsches Institut f€ur Normung(17)).
Flow curve and oscillatory strain sweep measurement
Once the sample was prepared following the procedure described above, rheological tests were conducted. Theflow curve measure-ment was performed at 25§ 1℃, with the shear rate increasing from 10¡ 8 s¡ 1up to 102 s¡ 1. The oscillatory strain sweep mea-surement was performed at 25§ 1℃, at a frequency of 1 Hz, with the shear strain sweeping from 10¡ 3% to 103%. Each type of
mea-surement was repeated at least twice. The repeatability of both flow curve and oscillatory strain sweep measurements was calcu-lated based on the deviation from the average value of the dupli-cated test results; seeTable 3.
Data process
Rheological output
Two representative results for bothflow curve and oscillatory strain sweep measurements are presented inFigure 3. As shown inFigure 3a, theflow curve measurement shows shear thinning of grease under continuously increasing shear. A zero-shear viscosity h0D 8:9 £ 105Pa¢s was obtained using the Cross
modelfit (Cross(19). InFigure 3b, the storage modulus G0, loss modulus G00, and crossover stress tcare obtained from the plot
directly and the yield stress ty¡ OSC was calculated using the
method described by Cyriac, et al.(20).
Entropy generation calculation
The entropy generated per unit volume during aging (Sg) will
be calculated based on the estimated frictional energy generated by the grease and recorded temperature during the aging pro-cess following the approach proposed by Rezasoltani and Khonsari(11).
If chemical reactions are neglected, the mechanical degrada-tion of grease is such a slow process that“the major portions of the system are in homogeneous states that change slowly enough with time” (Rezasoltani and Khonsari (21), p. 3). In this case, the entropy generated is equal to the accumulated energy divided by the aging temperature, which is produced through the absorption of heat (Tolman and Fine(22)).
The recorded temperature showed that during aging, the variation in aging temperature was smaller than 2℃ and the change in the system’s internal energy was negligible compared to the energy accumulated during aging. Therefore, the accu-mulated energy is equal to the work exerted on the grease; that is, the integration of the grease frictional torque and the rota-tional speed over the aging time (Rezasoltani and Khonsari
(21). The entropy generated per unit volume Sg can thus be
expressed as SgD work=Va TemperatureD
R
Torque¢ vdt Temperature£ Va: ½1Here, v is the input rotational speed, t is the aging time, and Vais the grease volume inside the Couette aging rig. In the
cur-rent rig, the torque generated by the grease cannot be recorded directly and is calculated using the shear stress t acting over the area 2pRh at a distance R from the central axis (Godec, et al.(23):
TorqueD 2pRht ¢ R D 2pR2htD 2pR2h_ga h j_ga: ½2 Here, _ga is the applied shear rate presented inTable 2and h j_ga is the grease’s apparent viscosity at the aging shear rate. The geometrical notations in Eq. [2] are shown in Figure 1. During the aging process, the grease’s apparent viscosity is not constant. To obtain the viscosity at the aging shear rate (hj_ga), Figure 2.Loading and preshear procedure.
Table 3.Repeatability of the rheological measurements.
Deviation Flow curve measurement Oscillatory strain sweep Li/M § 7:1% of the mean § 7:8% of the mean Li/SS § 4:6% of the mean § 10% of the mean
grease samples were collected after each aging period. Flow curve measurements were performed on these samples, from which the hj_gawas estimated from a Cross modelfit. An exam-ple of this hj_gais displayed as the aging point inFigure 3a.
In this way, the torque during the aging process can be cal-culated periodically. As an illustration, Figure 4 shows an example of the torque distribution where Li/M was aged at 83 rpm for 200 h. The accumulated energy and the correspond-ing entropy generation density can be calculated by integratcorrespond-ing the torque distribution over the aging period using Eq. [1].
The work and entropy induced by the 500 strokes mixture within the grease worker was calculated based on Rezasoltani and Khonsari’s(11)method: the entropy generated inside the grease worker is equal to the applied work Wgw divided by
the ambient temperature Tgw, where Wgwis the product of the
average load and the piston distance for one stroke multiplied by the number of strokes (500).
The average load F for one stroke (both tension and com-pression) during the pre-shearing, recorded by a load cell mounted beneath the cylinder of the grease worker, was FD 8:75 N for fresh Li/M and F D 6:20 N for fresh Li/SS. The pis-ton displacement for one full stroke was measured as LpistonD 5:68£10¡ 2m. The volume of the grease sheared
inside the cylinder was VgwD 1:23 £ 104 mm3and the ambient
temperature TgwD 25℃ D 298 K. The entropy generation per
unit volume during the 500 strokes is thus calculated as
SgwD
Wgw=Vgw
Tgw D
500 F¢ Lpiston
Vgw¢ Tgw : ½3
For Li/M, SgwD 6:8£10¡ 5J=mm3K; and for Li/SS,
SgwD 4:8 £ 10¡ 5J=mm3K.
In addition, the pre-shear procedure before the rheological measurement creates entropy. As specified in the Preparation section, the grease sample will be pre-sheared at _gpsD 100 s¡ 1
for 60 s at 25℃. Based on the approach of Rezasoltani and Khonsari(11), the entropy generation density during the pre-shear procedure Spswithin the rheometer can be expressed as
SpsD _gps¢
R
tpsdt
Tps ; ½4
where _gps is the shear rate applied during preshear
(_gpsD 100 s¡ 1), tps is the shear stress recorded during
pre-shear, t is time, and Tps is the temperature during preshear
(controlled at TpsD 25℃).
For fresh Li/M, SpsD 1:0 £ 10¡ 5 J=mm3K and for fresh Li/
SS, SpsD 5:2£10¡ 5 J=mm3K.
The entropy generation density during the sample prepara-tion—that is, SgwC Sps, is at least 100 times smaller than that
generated during the aging test and can therefore be neglected. For the following study, only the entropy density generated during the aging tests Sg will be taken into account.
Aging mechanism investigation
The microstructure of the grease thickener is studied using atomic force microscopy (AFM) in dynamic tapping mode, which is widely applied on soft biological samples (Hoefnagels
(24)) and greases (Sanchez, et al.(25); Paszkowski and Olszty n-ska-Janus (26)). The advantage of AFM over conventional scanning electron microscopy and transmission electron microscopy is that the soap structure can be observed without the need to remove the oil (Hurley and Cann (27); Delgado, Figure 3.Typical rheological output obtained from an aged sample: (a)flow curve test and (b) oscillatory strain sweep test.
Figure 4.Calculated torque distribution for LiM aged at 83 rpm for 200 h.
et al. (28)). In addition, the sample preparation is limited to smearing a small volume of grease on aflat glass plate.
Results and discussion
Verification of chemical reaction
The FTIR spectra (limited wave number range of 4000–650 cm¡ 1) of fresh and aged Li/M are presented inFigure 5. As an inhomogeneous material, grease thickener is not evenly distrib-uted, resulting in the amplitude variation at zone 3500–3230 cm¡1 (-OH bond), 1580 cm¡ 1 (COO¡ asymmetric stretch), and 1459 cm¡ 1(-CH deformation), which indicates the differ-ence in thickener concentration. However, no extra peaks are found between the fresh and aged samples’ spectra; therefore, based on the detection accuracy of the current FTIR device, chemical reactions are not observed during the aging tests. A similar result was found for Li/SS. This was to be expected, because the maximum value of the recorded temperature dur-ing agdur-ing was low for the lithium-thickened greases (52℃ for Li/M and 48℃ for Li/SS). Hence, the entropy calculation approach from Rezasoltani and Khonsari (11)can be applied (Rezasoltani and Khonsari(21)).
Thermodynamic characterization of grease mechanical degradation
In this section, the results from aging grease in the Couette aging rig will be presented and compared to those found by Rezasoltani and Khonsari(11). As specified in the Methodology section, we used the same shear rates and the same definition of entropy as Rezasoltani and Khonsari(11). However, they used the net penetration value as a response parameter, a measure of the consistency of a grease sample. To compare the current rhe-ological values to Rezasoltani and Khonsari’s (11) results, a relationship between the penetration value and the yield stress was applied (Spiegel, et al.(29)):
ty¡ HBD 3 £ 1010¢ D¡ 3:17; ½5
where ty¡ HB is the yield stress obtained from the flow curve data based on Herschel-Bulkley model (Spiegel, et al.(29), and D is the cone penetration depth (10¡ 1mm).
Rezasoltani and Khonsari (11) did not use the standard ASTM method to measure the penetration depth. However, it is assumed that they scale similarly. As listed in Table 2, 15 samples were prepared and examined for the effects of each
Figure 6.Comparison of penetration values against entropy generation density during the aging process: (a) reproduced from Rezasoltani and Khonsari(11)and (b) results from the current study (Li/M).
650
1000
1.500
2.000
2.500
3.000
3.500
4.000
-0.05
0
0.05
0.1
0.15
0.2
0.25
Wave nummber [cm
-1]
Ab
so
rb
an
ce
Spectra of fresh Li/M Spectra of Li/M aged for 5hr Spectra of Li/M aged for 25hr Spectra of Li/M aged for 50hr Spectra of Li/M aged for 100hr Spectra of Li/M aged for 200hr
type of grease on aging. The calculated penetration depth of Li/ M against entropy generation density is presented inFigure 6b, together with the data rebuilt from the results of Rezasoltani and Khonsari(11)(Figure 6a).
In the present case (Figure 6b), a linear relationship can be observed in the early stage of aging process; that is, Sg< 0:05 J=mm3K. After this, the degradation behavior changes.
Again a linear behavior is observed but with a different slope. The results from Rezasoltani and Khonsari(11)also show a deviation from the linearfit at higher values of entropy density. However, this was less pronounced. The aging behavior shown inFigure 6b
can be translated into a fast deterioration phase at the early stage and a slower deterioration phase afterwards.
Figure 7and Figure 8show the variation in the zero-shear viscosity with entropy generation per unit volume for Li/M and Li/SS, respectively, when subjected to three different shear rates. Again two phases can be seen: a progressive degradation in thefirst stage followed by a rather slow deterioration after-wards. This agrees with the penetration depth variation against entropy generation per unit volume illustrated inFigure 6. Sim-ilar trends were also found from the literature survey (Plint and Alliston-Greiner(30); Spiegel, et al.(31); Kuhn(32)).
The viscosity versus entropy generation trend of lithium-thickened grease is similar to the viscosity versus shear rate (shear thinning) behavior of lubricating greases. At relatively low shear (or energy), the grease has an initial high viscosity (here indicated as hi). However, when mechanical degradation starts, the sample begins to soften and after being subjected to a certain amount of entropy (SgD 0:05 J=mm3K), the viscosity
levels out again with a weak degradation rate.
Therefore, the formula of the Cross equation (Cross (19)), which is used to describe shear thinning behavior, was borrowed to describe the relationship between the variation in zero-shear viscosity and the entropy generation density during aging:
hoD hi¡ h1
1C K ¢ Sgm
C h1; ½6
where hi is the initial zero-shear viscosity for fresh samples, and
h1 is the viscosity for infinitely long shearing, which is calculated using Batchelor’s equation, h1D hbð1C 2:5’ C 6:2’2Þ (Barnes,
et al.(33); the base oil viscosity hband the phase volume ’ are
tab-ulated inTable 1; Sgis the generated entropy per unit volume
dur-ing agdur-ing, K is the coefficient of degradation, and m is the exponent of degradation.
Equation [6]fits the data obtained from all three shear rates very well (R2D 0:99); seeFigure 7andFigure 8. This model has
also been applied to the other three selected rheological proper-ties: storage modulus G0, crossover stress tc, and yield stress
ty¡ OSC from the oscillatory test in the form of Eq. [7], called here the grease aging equation:
YD Yi¡ Y1
1C K ¢ Sgm C Y1; ½7
where Y represents the rheological properties, Yirepresents the
initial rheological value for fresh grease, Y1 represents the sec-ond-stage value for the long time–aged sample, and K and m are the coefficient of degradation and the exponent of degrada-tion, respectively.
Each grease will have its own aging master curve, or grease aging equation, with its specific parameters; seeTable 4.
Grease aging mechanism
The generation of entropy demonstrates a dissipative process, which brings disorder to the system and, in this case, probably the collapse of the grease’s microstructure. According to the lit-erature, the consistency and rheological properties of fibrous structured greases are closely related to the geometry and distri-bution of the network structure formed by thefibers (Bondi, et al.(4); Moore and Cravath(5); Sanchez, et al.(25); Yoshiyuki, et al.(34)).
As shown inFigure 6b, there are two aging phases with dif-ferent degradation rates. According to Bryant, et al. (35), the entropy generation rate is closely related to the system degrada-tion rate. Here the system degradadegrada-tion rate is measured by the changes in rheological properties per unit of time Figure 7.Zero-shear viscosity variation versus entropy generation density for Li/M.
Figure 8.Zero-shear viscosity variation versus entropy generation density for Li/SS.
Table 4.Parameters for the grease aging equation.
Yi Y1 K m R2 h0ðPa¢ sÞ Li/M 1:1 £ 107 0:34 4:5 £ 103 1.9 0.99 Li/SS 5:7 £ 106 0:11 5:0 £ 10 0.89 0.99 G0ð ÞPa Li/M 9:0 £ 104 2:2 £ 103 5:9 £ 103 2.2 0.99 Li/SS 8:0 £ 104 9:3 £ 103 1:0 £ 103 1.6 0.98 tcð ÞPa Li/M 1:2 £ 103 1:0 £ 102 4:0 £ 103 2.2 0.99 Li/SS 7:2 £ 102 2:7 £ 10 4:0 £ 102 1.2 0.99
ty¡ OSCð ÞPa Li/M 70 3 1:8 £ 10 1.1 0.91
Li/SS 50 6 6:6 £ 102 1.9 0.95 TRIBOLOGY TRANSACTIONS 67
(macroscopically) and the change in the thickener network (microscopically). The entropy generation rate per unit of vol-ume can be expressed as
_SgD
Torque£ v=Va
Temperature : ½8
The entropy generation rate per unit volume and the degra-dation rate of the zero-shear viscosity h0 for Li/M during the aging tests are plotted against aging time in Figure 9. AFM measurements of the grease were taken at different points in time and are also shown inFigure 9. The cartoons are interpre-tations of the AFM pictures, which will help in the explanation of the results.
Figure 9 shows the entropy generation rate, rheology, and microstructure of Li/M during aging. Although only the degradation rate for zero-shear viscosity is shown, the plots for crossover stress, storage modulus, and yield stress are very similar. The thickener microstructure for fresh Li/ M is visualized as a twistedfibrous network where the fibers are typically 0.1–0.2 mm wide and up to 3 mm long. Initially the sample shows high consistency. When the mechanical degradation is initiated, energy is dissipated into the system (high _Sg), disrupting the network crosslinks and aligning the
fibers (as presented in the first two cartoon and AFM results). This results in a fast degradation in grease properties. This stage can be characterized by the coefficient of degradation K
(values of K are listed inTable 4) and it ends when thefibrous network becomes fragmented (until 50 h, as shown in
Figure 9).
After this fast degradation stage, the aged sample becomes a mixture of particle-like microfragments of thickener (with an average length of 0:1 mm) dispersed in the oil, and the deterio-ration process slows down (the degradation rate is approaching zero after 100 h of aging; seeFigure 9). Such behavior suggests that once the fibrous structure is completely destroyed, the grease rheology will become stable. With the deceleration of the aging process, the entropy generation rate becomes smaller and remains constant.
However, the existence of microfragments (or ‘nano size thickenerfibers’ termed by Yoshiyuki, et al.(34)) still gives the aged grease a higher consistency compared to that of the bled oil (see the second-stage values Y1 for the aged samples pre-sented in Table 4). According to the R2F bearing tests per-formed by Cann, et al.(9), small volumes of viscous liquid were found in the cage pockets, which was assumed to lubricate the rolling track; infrared spectroscopy showed that this lubricant was a mixture of oil and thickener, and more viscous liquid was found along with the running. Such viscous liquid can be con-sidered as the aged grease at the second stage, where it has both better flowability compared to the fresh grease (for lubricant replenishment) and higher viscosity compared to the base oil (forfilm construction).
Spiegel, et al. (31) described the fragmented thickener as spherical particles and modeled the second-stage Figure 9.Aging of Li/M; cartoon and AFM results for different aging stages.
mechanical aging using a W€ohler curve. Their theory sug-gests that when subjected to continuous shear, these spher-ical particles start rolling and the governing aging mechanism is fatigue. Considering the results shown in
Figure 9, in the second stage, the thickener structure has become fragmented during the second phase and the grease ages at a slower rate compared to the first aging phase. Currently, the grease rheology during aging is assumed to end up at an infinite value Y1 as shown in
Table 4and Figure 7 andFigure 8. A similar aging mecha-nism is also observed for Li/SS. To confirm Spiegel et al.’s
(31)theory, prolonged aging tests will be needed. Conclusion
In this study, the mechanical shear degradation of lithium-thickened grease was evaluated using an in-house-developed aging rig and a commercial rheometer. It was found that this grease loses its original consistency during aging and shows a two-phase aging behavior. In thefirst phase, primarily reorien-tation and breakage of the thickener network take place, result-ing in a progressive drop in the grease’s rheological properties. After this, the aging is dominated by the breakage of smaller fiber fragments, and the grease degrades at a much slower rate (currently considered stable). A grease shear aging equation (Eq. [7]) was introduced to describe such two-phase behavior. By making use of the entropy concept, this equation is capable of covering the change in grease’s rheological properties when aged at different shear rates. This aging behavior is closely related to the entropy generation rate and the change in the thickener network during the aging process: due to breakage of the thickener structure, grease degrades, and the aging rate is positively correlated to the entropy generation rate. According to Rezasoltani and Khonsari (11), the shear aging at various shear rates and temperatures can be described by a single (mas-ter) curve using the entropy concept. In the current study, the entropy concept was confirmed using aging at different shear rates. The current test rig does not make it possible to vary the aging temperature as well. It is therefore recommended to fur-ther study the impact of temperature on the shear aging behav-ior of grease.
Funding
The authors thank SKF Engineering and Research Centre for thefinancial
support of this work.
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